1. Field of the Invention
The present invention relates to a slope rate compensation circuit, method thereof and pulse width modulation (PWM) boost converter circuit, and more particularly, to a slope rate compensation circuit, method thereof and pulse width modulation boost converter circuit using a constant increasing slope rate.
2. Description of the Related Art
FIG. 1 shows a known pulse width modulation boost converter circuit 1, which comprises a boost converter circuit 10, a pulse width modulator 11, an oscillator 12, an error amplifier 13, a voltage divider 14 and a compensation circuit 15. When the pulse width modulation boost converter circuit 1 is activated, a reference voltage Vref is applied to the non-inverting input of the error amplifier 13. Meanwhile, the inverting input of the error amplifier 13 is connected to a feedback voltage VFB of the voltage divider 14 to define the magnitude of the DC output voltage Vout. The oscillator 12 outputs a pre-oscillating signal SOSC to the pulse width modulator 11 so as to generate a PWM signal SPWM. The PWM signal SPWM enables the transistor switch SW1, increasing the inductor current and storing energy in the inductor magnetic field. The output Eo of the error amplifier is compared with a constant-frequency sawtooth wave to determine the disabling duration of the switch SW1, so that the inductor current IL, generated by the input voltage Vin, flowing through the boost inductor L, is capable of intermittently charging the capacitor C11 and thus increases the DC output voltage Vout. The diode D11 limits charge current direction of the capacitor C11. When SW1 is disabled, D11 is forward biased.
A PWM boost converter circuit is applied to either the circuit mode PWM control or to voltage mode PWM control. The current mode PWM control uses inductor current as a feedback signal, and thus exhibits a superior loop performance over the voltage mode PWM control. In addition, without a leading compensation, the current mode PWM control has a phase margin 90 degrees more than does the voltage mode PWM control, and thus is more stable.
Generally, if the duty ratio of a PWM switching regulator is greater than 50% in the current mode PWM control, it is necessary to conduct a slope rate compensation. The compensated slope rate ma must satisfy the criteria
            m      ⁢                          ⁢      2        2    <  maso as to avoid unstable and inefficient situations. When ma is equal to m2, it represents the optimal compensation slope rate, where −m2 is the slope rate of the inductor current when the transistor switch SW1 is disabled.
FIG. 2 shows a waveform of the inductor current IL of the PWM boost converter circuit 10. As shown in FIG. 2, the slope rate m1 of the inductor current is
      V          i      ⁢                          ⁢      n        Lwhen the transistor switch SW1 is enabled, the slope rate −m2 of the inductor current is
            V      out        -          V      IN        Lwhen the transistor switch SW1 is disabled, and
      V    out    =            V              i        ⁢                                  ⁢        n              ×                  1                  (                      1            -                          duty              ⁢                                                          ⁢              ratio                                )                    .      Therefore, if ma is equal to m2,
  ma  =                    V                  i          ⁢                                          ⁢          n                    L        ×                            duty          ⁢                                          ⁢          ratio                          (                      1            -                          duty              ⁢                                                          ⁢              ratio                                )                    .      The primary difficulty in determining proper slope rate compensation is that the input and output voltages of the PWM boost converter circuit 10 are variable, as is the duty ratio; therefore it is not easy to locate a corresponding variable slope rate compensation.
U.S. Pat. No. 6,611,131 discloses a piecewise-linear slope rate compensation, which adopts resistors to detect inductor current and uses a BJT transistor to conduct the slope rate compensation. First, it results in a greater power consumption. Second, the level of voltage clamping shifts with the voltage source, so the design is more complicated. Third, the current limit is achieved by clamping the feedback signal of the inductor current, but the current limit level is distorted after a summation is performed with the slope rate compensation signal.
FIG. 3A shows output voltage waveforms of high duty ratio and low duty ratio without slope rate compensation, and FIG. 3B shows output voltage waveforms of high duty ratio and low duty ratio with slope rate compensation. As can be seen in FIG. 3B, the output voltage of high duty ratio with slope rate compensation reaches the circuit limit level more easily than the output voltage of low duty ratio with a slope rate compensation voltage since a larger slope rate is added.
U.S. Pat. No. 6,522,116 discloses another slope rate compensation technique, which must detect the variation of the output voltage before a slope rate compensation is conducted. However, detecting the variation of the output voltage itself is a difficult task. Since generally, the monolithic IC has no access to the output voltage, if the detection circuit exhibits an inaccurate performance due to oversimplification, the overall performance is decreased. In addition, this prior art adopts a large resistor to perform a linear effect, thus resulting in excessive power consumption.
U.S. Pat. No. 5,717,322 discloses another slope rate compensation technique. Since its compensated slope rate is constant, its performance varies with the duty ratio.
Because the known prior arts have these drawbacks listed above, it is necessary to design an easily-manufactured, power-saving slope rate compensation technique whose slope rate is variable with the working duty ratio.